Manufacture of optical glass



1942- F. J. DOBROVOL NY 2,294,077 I I MANUFACTURE OF OPTICAL GLASS Filed Dec. 23, 1940 1 N820 MOLAR RATIO az a 2 3 IN V EN TOR.

w m m T A COATING UR PLASTlC Search Room Patented Aug. 25, 1942 MANUFACTURE OF OPTICAL GLASS Frank J. Dobrovolny, Ransomville, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application December 23, 1940, Serial No. 371,218

6 Claims.

This invention relates to the manufacture of glass, and more particularly to the production of high grade optical glass.

This application is a continuation in part of proved quality having fewer imperfections in the finished glass. Another object is to provide a method for decreasing formation of striae and similar imperfections in the manufacture of 01)- my co-pending application, s m n Number 5 tical glass. Still other objects will be apparent 120,613, filed January 14, 1937, now Patent No, from the following description of my invention. 2,230,199. The above objects are obtained in accordance Optical glass ordinarily is soda glass of high with t p s n inv n o b po at in quality which is suitable for the manufacture of e glass batch a certain limited amount of so- 1ens ri and Similar optical devices, Qptidiurn monoxide, as hereinafter described. In cal glass must be made of ingredients of highest practicing my invention, after the formula or purity, so as to obtain a substantially colorless making Optical glass h s been selected, I revise and highly tra s ar nt glass, that formula by substituting for part of the alkali The compositi n of optical glass will va d metal carbonate a stoichiometrical equivalent of pending upon the particular use for which it is sodium monoxide (N320). I have discovered that de i ed F example by Varying th o iif the amount of sodium monoxide used is such tion, the optical properties of the glass such as that the molar ratio of sodium monoxide to the refractive index absorption spectra, and the like total alkali metal carbonate in the glass batch is may be varied as desired, However, for many greater than about 0.121 but less than 0.321, the purposes a conventional soda-lime glass often is resulting ss W ave fewer i p t o s than suitable for optical purposes if it is of high ss of d tica mposition which was made quality. In other cases, ingredients such as mag- Without n di s dium m n x de in the glass nesia, borax, lead oxide, or the like may be inbatch. I have further discovered that in order cluded in the glass batch formula to obtain speto obtain s result, it s necessary to ave the oifio optical properties, In any event one or relative DIODOltlOIlS of sodium monoxide and almore alkali metal carbonates, together with silica, al metal Carbonate Within the ratio expressed form the basis for the mixture of ingredients to ov Preferably I maintain the molar ratio of be fused to make the optical glass. sodium monoxide to total alkali metal carbonate Pieces of optical glass suitable for making i in the glass batch within the range Of 0.1311 to prisms, lenses, and the like also must have a high 0.26:1. Within this preferred range. the resultdegree of uniformity and be entirely free from i s ass is t a d na i y fr e fr s a as gas bubbles, striae, and similar imperfections. In bubbles, d similar i perfections. order to obtain such pieces of glass, optical glass The pp d d aw ng is a graph showing the is made by fusing silica, soda ash and other suitresults of adding o u o d t0 the glass able glass making ingredients in clay fusion pots. 3 batch n making Op c g ass, Curves A and B The fusion of the ingredients is carried out with n t e d aw ng are drawn through points Plotted the utmost care to avoid contamination of the fr m actu l p s- Curve C represents melt. After fusing, the glass batch is kept molten the mean of curves A and B. In plotting these for a relatively long period of time and well curves, glass samples made from the fusion of stirred, so as to eliminate all gas bubbles, and batches containing varying amounts of sodium inhomogeneities so far as possible. The glass is monoxide and sodium carbonate (all samples then cooled and annealed without removing it having identical composition as determined by from the fusion pot; and finally, on cooling furanalysis) were examined by a number of obth'er, the glass cracks and breaks up into fragservers and rated as to the presence or absence ments, accompanied by destruction of the clay of striae. Those samples which were of the pot. In spite of the most careful operation the highest quality in this respect were given a rating resulting pieces of glass will have many imperof l and those of poorer quality were given a fections, especially striae formed by incomplete rating represented by a higher number as is more solution or reaction of the ingredients. It is, fully explained in the examples below. These therefore, necessary to examine each piece or ratings were plotted as ordinates and the molecufragment to select those which are free from imlar ratio of the sodium monoxide and the total perfections. As a result of this operation usualkali metal carbonates from the glass melt were ally only a relatively small portion of the original plotted as abscissae. glass ingot is selected as suitable for the manu- From this graph it will be noted that while the facture of optical devices and usually it is difiireplacement of the alkali metal carbonate by cult to find many pieces suitable for making large sodium monoxide in the glass batch progressively lenses and prisms, for example, lenses 4 or more increases the quality of the resulting glass up to in diameter. a certain point, a further increase of the sodium An object of the present invention is a novel monoxide content of the batch tends to rapidly and improved method for making optical glass. A further object is to make optical glass of imdecrease the quality of the product. I have found that if too much sodium monoxide is used the quality of the resulting glass is no better than if no monoxide at all wer used, and in some cases even poorer.

In one method of .practicing my invention I may make a finely divided mixture of glass making ingredients including a suitable form of silica such as sand, a calcium compound such as high grade lime or precipitated calcium carbonate, soda ash or other suitable form of sodium carbonate, and a limited amount of sodium monoxide, as described above. Other ingredients may :be added to the mixture as desired, for example, an oxidizing agent such as sodium nitrate. In addition to soda ash, another alkali metal carbonate, for example, potassiiun carbonate may be added. In any event, the molar ratio of sodium monoxide to the total alkali metal carbonate in the mixture will lie within the range of 0.1:1-0.3:l, and preferably between 0.13:1 and 0.26:1. Obviously, the relativ amounts of alkali metal carbonate, sodium monoxide, and other alkali metal compounds such as sodium nitrate, borax or the like are added in such amounts as to obtain the desired content of alkali metal oxide as determined by analysis of the finished glass. Such a mixture, in accordance with my invention, is fused to form molten glass and then is refined, cooled, and annealed and subsequently treated in the conventional manner.

EXAMPLE A series of 8 mixtures of glass making ingredients were made up so as to contain sand, calcium carbonate, sodium carbonate, potassium carbonate and, with the exception of two mixtures, sodium monoxide. The amounts of the various sodium and potassium compounds and other ingredients in these eight mixtures were proportioned so as to produce on fusion glasses having substantially identical compositions. The composition of the eight mixtures is shown in the following table:

Table I (The figures opposite the ingredients indicate parts by weight.)

The eight batches were fused in clay crucibles in the same furnace under identical conditions. The operations of fusion, refining, cooling and annealing of the eight batches were carried out under substantially identical conditions. The resulting pieces of glass from each batch were independently examined by at least four observers and each observer rated the batch examined as No. 1 (best), No. 2 (second best), No. 3 (third best) or No. 4 (poorest). These ratings were based solely upon the presence or absence of striae in the glass. From these independent ratings an average rating for each ingot was determined. For example, in the case of one ingot, 50% of the observers rated it as No. 3

and 50% rated it as No. 4; hence the rating for this ingot was calculated to be 3.5. The results of this examination and rating is tabulated below:

Referring to the appended drawing these experimental results were plotted as curves A and B which were explained above. The average of these ratings was then determined and plotted as curve C as shown in the appended drawing.

I claim:

1. The process for making optical glass which comprises fusing a mixture of glass making ingredients containing at least one alkali metal carbonate, and a limited quantity of sodium monoxide in such proportion that the molar ratio of sodium monoxide to total alkali metal carbonate is less than 0.3 to 1 but not less than 0.1 to 1.

2. The process for making optical glass which comprises fusing a mixture of glass making ingredients containing at least one alkali metal carbonate, and a limited quantity of sodium monoxide in such proportion that the molar ratio of sodium monoxide to total alkali metal carbonate lies within the range of 0.13:1 to 0.26: 1.

3. The process for making optical glass which comprises fusing a mixture of glass making ingredients containing sodium carbonate and potassium carbonate and a limited quantity of sodium monoxide in such proportion that the molar ratio of sodium monoxide to total alkali metal carbonate is less than 0.3 to 1 but not less than 0.1 to 1.

4. The process for making optical glass which comprises fusing a mixture of glass making ingredients containing sodium carbonate and potassium carbonate, and a limited quantity of sodium monoxide in such proportion that the molar ratio of sodium monoxide to total alkali metal carbonate lies within the range of 0.13:1 to 0.26:1.

5. The process for making optical glass which comprises fusing a. mixture of glass making ingredients containing sand, calcium carbonate, sodium nitrate, sodium carbonate, potassium carbonate and a limited quantity of sodium monoxide in such proportion that the molar ratio of sodium monoxide to total alkali metal carbonate lies within the range of 0.13:1 to 0.26:1.

6. The process for making optical glass which comprises fusing a mixture of glass making ingredients substantially as follows:

Parts by weight Silica 3600 Calcium carbonate 800 to 1100 Sodium nitrate 500 to 700 Sodium carbonate 630 to 680 Sodium monoxide 70 to The molar ratio of sodium monoxide to total alkali metal carbonate in said mixture lying within the range of 0.13:1 to 0.26:1.

FRANK J. DOBROVOLNY. 

